The present invention relates to a microwave heating system for recovery of petroleum from petroleum impregnated media, and more particularly to the recovery of extractable organic or carbonaceous values from petroleum impregnated porous media such as oil shale, oil and tar sands, heavy oil reservoir deposits and residual heavy oil pools, e.g. previously subjected to primary oil well drilling extraction, and the like, by the use of microwave or high frequency RF, i.e. radio frequency, radiation energy for in situ heating, preferentially of the liquifiable and gasifiable constituents such as hydrocarbons present in the pores of the media.
Hydrocarbons are found in varying compositions in various underground formation deposits, such as kerogen in oil shale and bitumen in oil sands and tar sands. Likewise, heavy oils with a high viscosity are found in reservoirs located within certain rock or sand formations. These types of hydrocarbons found in such deposits require heat to effect either thermal or chemical change for release and recovery of the desired carbonaceous constituents. Certain known processes require both heating and chemical change to attain such recovery.
However, attempts to recover, in situ, petroleum from oil bearing media have been limited to poorly controllable and inefficient bulk heating or gross heating recovery methods using primarily steam or hot water to heat the media for causing the oil constituents to become sufficiently flowable to be entrained in the steam or hot water and removed in admixture therewith, whereupon the oil has to be separated from the mixture once raised from the underground site to the surface.
These attempts typically require the steam or hot water to pass from the surface down a bore hole to the site of extraction at the underground level of the stratum in question, and to be pumped back to the surface as a mixture with the entrained oil constituents. Since the heating of the underground site is primarily by way of conduction heat transfer, both the desired oil constituents and the entire surrounding rock formation have to be heated in bulk, and the system is beset with pronounced Btu (British thermal unit) heat loss through dissipation during travel of the steam or hot water along the pronounced distances of the bore hole between the surface steam generator or hot water heater and the underground deposit extraction site, in some cases amounting to many thousands of feet of separation.
As a consequence, the overall energy requirement for inefficiently providing such bulk heat at the underground extraction site is so great that these known methods are generally considered commercially impractical and economically unfeasible for industrial scale production purposes.
Even where open pit or strip mining and underground mining via open shaft and gallery arrangements, e.g. using the room and pillar method, are conducted, the attempts have not been successful since on the one hand, the landscape is inherently disturbed by open pit or strip mining and the operation must conform to governmental environmental regulations and on the other hand, the mined oil bearing rock media must be brought in its entirety from the gallery through the shaft to the surface. In both cases, the entire mass of the mixed oil bearing rock media must be subjected to crushing and then retorting in a closed vessel.
Such closed vessel retorting offers poor control and consumes large quantities of energy for heating the rock, as well as the oil, likewise by bulk heating, in most cases with the operation being carried out in the presence of air.
In the usual retorting operation, air is used to burn a portion of the desired oil content by direct combustion therewith in the retorting vessel so as to provide the necessary heat. This expedient not only consumes oil but also results in a gaseous fraction in which the valuable gasified oil constituents are admixed and thus diluted with contaminating gaseous combustion products such as carbon dioxide.
Moreover, where incomplete combustion is carried out, i.e. with the use of smaller amounts of air in proportion to the carbon rich and/or hydrogen rich constituents in the oil, in addition to water formation the retorting leads to the production of carbon monoxide, rather than carbon dioxide, per the well known endothermic reaction by which any formed carbon dioxide is reduced in the presence of excess carbon and/or hydrogen, relative to the attendant oxygen content, to carbon monoxide, depending on the attendant combustion conditions. This renders the heating system nonuniform and causes a loss in heat values to the extent that carbon monoxide, of comparatively low Btu value, is so formed in extra amounts than otherwise.
On the other hand, where the retorting is carried out in the absence of air, i.e. by indirect heat exchange, the bulk heating is even more inefficient, taking longer and thus consuming even more energy.
Present day consensus is that the United States must develop realistic alternative energy sources if the nation, and indeed the industrialized world in general, are to survive the portended future energy crisis.
One possible solution to the energy shortfall facing the United States in particular is the development of the vast oil shale deposits found especially in the States of Colorado, Utah and Wyoming. For instance, oil shale of the Eocene Green River Formation in adjoining corners of these three states is estimated to contain 1.5 trillion barrels (bbls) of potential oil in place. This oil shale has low sulfur and high nitrogen content compared to petroleum as currently obtained.
Present oil shale activity in this regard is essentially experimental and its production insignificant due to the above noted drawbacks. Although many recovery methods are under study from time to time, costs have always been a deterrent, and environmental and/or technical barriers loom as insuperable.
It is estimated that present day oil shale recovery costs amount to from about $35 to $55 per barrel of produced oil, so that it is easy to see why present economics for developing otherwise readily available oil shale deposits are dubious.
As to surface retorting or fired methods, these not only involve the costs for mining the shale but also for crushing the rock to a retortable size, and then carrying out the actual retorting. Underground mining also includes the actual cost of physically bringing the mined rock to the surface through the open shaft.
Certain proposed underground mining methods contemplate the gallery or room and pillar method, but have been initially confined to shales of the mahogany zone that are 1500 feet or less below the surface, and that average 30 gallons per ton (g pt) or more in large beds 30 to 90 feet thick. These limitations are imposed by the costs currently encountered in underground mining.
Underground mining, and even surface mining by way of open pit or strip mining technique, involve measures that require at least five handlings of the shale, e.g. for physically extracting or mining it, hauling it, crushing it, retorting it, and disposing of the solid spent shale rock residue. These collectively constitute a significant collateral cost to shale oil production.
Moreover, the environmentally acceptable disposal of the solid spent shale rock residue from the retorting, which represents about 80-85% of the weight of the shale, is itself costly, and is in addition to the costs of rehabilitating the ground surface to meet governmental environmental regulations in the case of open pit or strip mining, in particular.
In fact, not all underground oil shale deposits lend themselves to mining, and recovery from these deposits is limited to in situ, or in place, methods. A typical example is an oil shale deposit of 625 square miles in the State of Wyoming that is estimated to contain over 200 million barrels of oil per square mile. Unfortunately, where this rich oil shale occurs, the deposits are vertically discontinuous alternating thin horizontal beds of rich and lean oil shale, rather than the more desirable vertically continuous or thick horizontal deposits. Hence, mining oil shale from this deposit is perforce economically unattractive, and recovery would only be practical with in situ or in place methods.
Heat, of course regardless of its source is essential in the processing of oil shale into shale oil whether by mining and then surface retorting or by in situ retorting.
The situation is the same where in situ retorting is used for treating tar sand deposits rather than oil shale. Vast tar sand deposits exist in the United States and Canada which contain very heavy viscous crude oil or bitumen. This bitumen must be heated to facilitate its removal. Present heating methods use surface heated steam to heat the bitumen, e.g. to 300.degree.-400.degree. F. (149.degree.-204.degree. C.), to make it less viscous and thus more readily flowable. Such heating by steam is dependent upon the conduction of heat between fluid molecules, and is subject to heat loss and inefficiency problems.
In fact, the bitumen once recovered from the tar sands deposit must be converted into a light sweet crude before it can be refined or even transported. during such conversion, the bitumen is broken apart thermally into smaller fractions and the resulting material then hydrogenated. This helps to make the material sweeter and lighter. The process is not unlike hydrogenating margarine, and requires carbon removal and the addition of hydrogen, but represents and afterprocessing burden on the overall operation.
North American tar sands deposits are estimated to hold more oil potentially than the entire Middle East, and exploitation of such tar sands deposits could help the industrialized West to achieve energy independence. However, of these vast "heavy oil" deposits, it is considered that only about 100 billion barrels could be recovered within the limitations of current technology and economic conditions. Improved technology would, of course, increase significantly that potential.
A third source of potential fossil energy in significant amounts is found in the still remaining petroleum deposits or heavy crude oil reservoir deposits or residual heavy oil pools previously subjected to primary oil well drilling extraction. These latter deposits which are located in subsurface reservoirs or pools of depleted or partially depleted oil wells, are what remain in exploiting our present main source of petroleum energy from "dome oil" wells. The primary recovery of this oil is effected by sinking wells into oil bearing formations and allowing the natural pressures within the oil impregnated strata to force the fluid into the well bore where it can be conveniently collected by pumping.
In some of these "dome oil" reservoirs and in partially depleted reservoirs there may not be enough natural pressure available to force the oil into the well bore at a sufficient rate to be economically profitable. In other reservoirs the oil flow may be retarded by the "heavy oil" and paraffin content of the petroleum that closes the natural flow channels of these underground crude oil reservoirs. Standard secondary recovery methods such as the injection of water, gas, air or a combination of these materials into the formation are used, as well as the application of heat energy by either chemical or electrical means. Hence, these are often referred to as "huff and puff" pool oil wells.
Where direct firing or in situ retorting of the oil or gas bearing formations in these "dome oil" reservoirs or "huff and puff" pool oil wells is used instead, it is found to produce a contamination of the crude petroleum or gases, and thus suffers from the same drawback as direct firing in the case of surface retorting of mined oil shale.
Chemical heating methods, like hot water and steam heating methods, have generally been unable to provide sufficient heat economically or satisfactory results. For the same reason, electrical resistance heating methods have proven unsuitable in that the transfer of heat to the oil bearing strata is primarily bulk heating or gross heating, accomplished by conduction. In all of these cases, the rate of conduction is low and the heat is continually drawn away from the oil bearing strata by the pumping of the heating oil. Chemically or electrically provided heat must also be expended to heat both the formation itself and the oil.
A particular problem with all conventional downhole heating methods which rely solely on heat conduction is that the heavier crudes, which require most of the heating, are the poorest type of thermal conductors among the crude oils. This aggravates the energy consumption in heating not only the oil but also the surrounding rock, since the rock is a poor thermal conductor as well as must be heated to the same extent as the oil, including the heavier crudes, before the temperature is sufficient consequent such bulk heating or gross heating to facilitate flow of the oil through the channels in the formation to the well bore.
The reason why oil shale requires the application of heat in order to produce oil is because the carbonaceous values contained in the oil shale rock are in the form of solid insoluble organic matter, and not oil. However, this solid organic matter will decompose to yield oil, when heated, i.e. when it is retorted, such oil being recovered in the form of oil vapors along with gas, e.g. non-condensible gaseous constituents admixed with the oil vapor constituents.
In this regard, oil shale has been described as a sedimentary rock with relatively high organic content, e.g. 30-60% volatile matter and fixed carbon, that yields an oil when heated. On the other hand, it does not yield oil when extracted with ordinary solvents. Typical oil shales may yield anywhere from 20-50 gallons of crude oil per ton (gpt), the oil constituents often being of a relatively unsaturated or olefinic character compared to the usual petroleum.
The organic oil yielding matter present in oil shales as solid insoluble matter is generally called kerogen. Kerogen is not considered a definite compound but has been described as a complex mixture of various complex compounds that varies from one shale species to the next, and usually exists as a soft brown powdery material that is at best only slightly soluble in ordinary organic solvents, and that may contain small proportions of nitrogen and sulfur constituents as well as oxygen, e.g. as hetero atoms. The porous rock matrix in which the kerogen is situated in oil shale usually contains associated free water and bound water of crystallization, e.g. where the rock consists of carbonates, silicates, aluminates, etc., often in conjunction with pyrites.
Kerogen in oil shale must be heated to high temperature before it pyrolyzes or decomposes. For this reason, in the case of surface retorting, the mined oil shale must first be crushed to reduce its size for more efficient exposure of the material to the heat. Despite significant world oil price increases, a primary reason why the known mining, crushing and retorting technique for recovering oil from oil shale has still not become commercially viable is because oil shale is a relatively lean ore.
Experience has shown that even a ton of relative rich oil shale of 25 gpt (but actually a lean ore at 0.0125 gallon per pound, i.e. 25/2000) will only produce about 0.6 barrel of oil, after expending elaborate efforts in the five handlings of mining, hauling, crushing and retorting the shale, and then disposing of the spent shale within environmentally acceptable guidelines, aside from the energy consumed in bulk heating of the shale to accomplish pyrolysis of the kerogen during the retorting.
The alternative of bulk heating of the oil shale in a surface retort is by burning or retorting it in place underground by direct combustion of a portion of the kerogen content with supplied extraneous air, but such is no less impracticable, aside from the contamination of the produced oil with combustion products and possible environmental hazards. This is because much of the oil shale encountered underground is nearly impermeable, despite its internal content of tiny pores containing the kerogen, and must first be mechanically broken up in order to permit the hot combustion gas to pass through it.
A satisfactory method to break up shale oil deposits in place has not been found. The present method involves blasting the rock and the removal of a portion of the rubble (about 25%) to allow for a fire-flow through the fractured deposit. Results are less than satisfactory because of the inefficient burning of the shale due to the non uniform size of the rubble.
Actually, oil shale deposits exist as planes or discontinuous deposits or beds of varied thickness at random levels along the underground formation, and each may be a relatively rich or a relatively poor oil shale plane or bed alternating with intervening planes or beds of barren rock.
Because of the nature of the particular porous media and its impregnated petroleum content, whether in the form of oil shale, oil sands, tar sands, heavy oil reservoir deposits, residual heavy oil pools, e.g. previously subjected to primary oil well drilling extraction, and the like, the manner in which the particular deposit of the porous media occurs in the underground formation, e.g. in lean and rich discrete beds, often of narrow seam height randomly disposed along the vertical course of the formation, and/or in deposits or reservoirs or pools of pronounced depth from the ground surface, and furthermore, because of the inefficiencies and cost of gross heating or bulk heating, whether by in situ heating using hot water or steam, or chemical or electrical heating means, or by in situ retorting or direct firing with supplied extraneous air, or by surface retorting using direct firing with supplied extraneous air or indirect heat exchange, normally preceded by crushing and followed by spent shale disposal, none of these known techniques has been commercially successful or competitive with petroleum obtained by the usual primary oil drilling methods from dome oil reservoirs and the like.
U.S. Pat. No. 4,193,448, issued Mar. 18, 1980 to Calhoun G. Jeambey, discloses and claims an apparatus, e.g. in the form of an elongated shell attached to the lower end of a pipe arrangement, for recovery of petroleum from petroleum impregnated media such as rock, shale and sands, and includes an electrically energized microwave generator and a guide for directing microwaves to a microwave dispersing chamber for heating the media, plus a plurality of holes in the shell for the inflow of heated petroleum into a petroleum chamber from the heated media. The apparatus is inserted into an opening, e.g. a borehole, in the media, then microwaves are dispersed into the media to heat the same, and the heated petroleum is recovered therefrom in the petroleum chamber via the holes. The system is safe, cost efficient and at least as fast as conventional methods for the recovery of oil from shale, while using substantially less energy than that required for conventional heating methods. In particular, there is no substantial alternation of the landscape nor appreciable environmental impact since the heating and recovery operations are conducted underground, i.e. at a downhole site in the borehole.
However, U.S. Pat. No. 4,193,448 does not disclose extensive or particularized details as to the actual process of extracting or recovering in situ the petroleum or oil from the impregnated media, let alone the pyrolysis production of both oil and gas, including that traceable to residual solid form carbon coke remaining after pyrolysis of kerogen, etc. to remove the initially generated liquid and gas constituents, while permitting molecular break down or "cracking" of the attendant hydrocarbon constituents to smaller molecules and particularly to increasing proportional amounts of noncondensible gases.